#define _CRT_SECURE_NO_WARNINGS 1
#pragma warning(disable:6031)
#include <iostream>
using namespace std;

int main() {

    int n;
    cin >> n;
    int sum = 0;

    for (int i = 0; i <= n; i += 2)
    {
        sum += i;
    }


    cout << sum << endl;

    return 0;
}